A circuit of this type is already known from DD 110983. The circuit has input terminals. Connected in series between the two input terminals are a resistor and a ZENER diode. A measuring voltage, to which the output voltage of the circuit is to be controlled, is picked off at the ZENER diode by means of a potentiometer. The control takes place by means of a voltage-discriminator transistor, the emitter voltage of which is at the base of a transistor, which drives a Darlington transistor arrangement. The measuring circuit lies here upstream of the final control element formed by the Darlington circuit in the direction of current flow. Connected downstream of the final control element in the direction of the current is a shorting circuit, which comprises a thyristor which shorts the output terminals when a predetermined output voltage is exceeded.
DE 4000674 A1 describes a “crowbar”, in which a voltage is picked off at two output terminals. If said voltage exceeds a threshold value, here too the thyristor is triggered, which causes a short-circuit.
DE 1936278 describes a shorting circuit with a trigger voltage generator.
A further shorting circuit is described by DE 102004025420 A1. There, a fuse is provided between a pair of input terminals and a pair of output terminals. Downstream of the fuse in the direction of the current there is a thyristor between the output terminal and the input terminal. This is triggered if either the output voltage or the output current exceeds a permissible threshold value. With the triggering of the thyristor, the output terminals are shorted. The resultant abruptly increased current causes the fuse to blow.
DE 102006003620, which is not a prior publication, describes a similar power limiting circuit. The threshold voltage is defined here by the breakdown voltage of a ZENER diode, which with a resistor forms a voltage divider bridge, which is used to pick off the trigger voltage for a thyristor which triggers in the case of a threshold current or a threshold voltage being exceeded at a measuring resistor and consequently causes a short-circuit, which results in blowing of the fuse.
To avoid a reversible or irreversible triggering of this aforementioned crowbar circuit, in the prior art control devices are connected upstream of the crowbar circuit. Typically, an electronic voltage or current limitation or voltage-current control is connected upstream of the crowbar, in order to limit the monitored voltage or the monitored current to a value below the threshold value of the crowbar. It is intended in this way to limit the voltage present at the crowbar or the current flowing through the crowbar to values that lie below the threshold value of the crowbar.
The threshold value of the crowbar is subject to tolerances, which depend for example on the breakdown voltage of the ZENER diode used. The switching threshold of the controlling or limiting circuit must lie below the minimum tolerance value to ensure that the monitored voltage can never reach the crowbar threshold value. The switching threshold of the control is likewise subject to component tolerances. Here, too, a ZENER diode is suitable as a threshold-defining component. The tolerances of the limiting circuit and the tolerances of the shorting circuit are independent of one another. This has the result that the overall tolerance range of the limiting circuit must be at a discrete interval from the tolerance range of the shorting circuit. This in turn has the consequence that the guaranteed output voltage that is present at the output terminals of a protective circuit according to the prior art is much lower than the permissible safety voltage that must not be exceeded as an output voltage.
It is therefore an object of the invention to provide measures by which the difference between the guaranteed output voltage and the safety voltage can be minimized.